As the pace of life accelerates, more people are demanding fast and reliable access to information, including audio and video content, from their content service providers. As a result, content service providers must find ways to update their systems to support the transmission of larger volumes of content at higher transmission rates. The rate of content transmission is typically described in terms of the bit rate of the connection or transfer. The bit rate is the number of bits that can be conveyed or processed per unit of time (e.g., gigabits per second, or Gb/s).
In fiber optic networks, where bits are conveyed via an optical signal that is propagated along an optical fiber, increasing the bit rate of content transmission is not as simple as pushing more bits per second through the fiber. As the bit rate is increased, for example from 2.5 Gb/s to 40 Gb/s, the sensitivity of the optical signal to attributes of the fiber is heightened. For example, deviations in the cross-section of the fiber and/or twisting of the fiber, which may have no significant effect on optical signals propagated at 2.5 Gb/s, may result in noticeable signal distortions at 40 Gb/s. Fiber attributes may include the optical signal-noise ratio (OSNR), chromatic dispersion (CD), polarization mode dispersion (PMD), pass band shape, and nonlinear effects. As a result, the fiber must be carefully characterized so that the fiber pathway can be tuned to accommodate the faster bit rate before the fiber system is put on-line.
Many fiber optic systems use Wavelength Division Multiplexing (WDM) to combine multiple optical signals on a single fiber, using different wavelengths of light to carry different signals. As an example, one optical fiber may have the capacity to carry eight signals on eight wavelengths, or channels. Most WDM systems are not fully populated, meaning that not all of the channels of a given fiber are carrying signals at any given time. However, using one of the available channels as a test channel for characterizing the fiber's attributes may cause interference with the optical signals carried on the live channels of the same fiber, thereby negatively affecting the quality of the transmission on those channels.
As a result, some have used the characterization of dark fibers, or fibers that carry no signal traffic on any channels, to estimate the properties of other, lit fibers in the same cable. However, although a dark fiber itself may be accurately characterized, the dark fiber may have properties that are slightly different than those of the other fibers, even in the same cable. In addition, as the demand on fiber optic systems increases, the number of dark fibers available for testing decreases, making statistical estimates less reliable.
Thus, there is a need for a system and method for characterizing lit optical fibers, such as in a WDM system, without adversely affecting the transmission performance of the fiber optic system.